The present invention relates to a process and apparatus for the production of synthesis gas, particularly for but not necessarily limited to, use in the production of hydrocarbon liquid fuels (e.g. using the Fischer-Tropsch (“F-T”) process), methanol (e.g. by catalytic hydrogenation of carbon monoxide), oxo-alcohols and dimethyl ether (“DME”).
Natural gas may be found in remote locations both on- and offshore. It is generally expensive and impractical to transport natural gas from its source to a distant processing plant. One solution is to convert the gas on-site to a valuable and easily transportable product. In this way, the value of the natural gas may be increased.
Natural gas may be converted to synthesis gas (or “syngas”) which is a mixture of carbon monoxide and hydrogen. Syngas may be converted to a solid or liquid synthetic fuel (“synfuel”) or converted to methanol, oxo-alcohols or DME. For optimum conversion in the F-T process, the ratio of hydrogen to carbon monoxide is preferably about 2 to 1. The conversion products have less volume per unit mass (i.e. have a greater density) than the natural gas. Accordingly, it is more economical to transport conversion products than a corresponding amount of natural gas.
Syngas may be produced using a heat exchange reforming (“HER”) process. A conventional two-step HER process may use natural gas as feedstock and employs a primary exothermic (or heat-generating) unit producing syngas, e.g. from natural gas and oxygen, coupled with a secondary endothermic (or heat-requiring) unit that uses at least a portion of the heat generated in the primary unit to produce further syngas, e.g. by a reforming reaction of natural gas and steam. In certain HERs, the syngas generated by the HER feeds the primary exothermic unit, while other HERs operate in parallel to the exothermic unit and augment the syngas production therein.
There are several methods of producing syngas from natural gas. Examples of these methods include:
(a) Steam-methane reforming (“SMR”) which uses an endothermic catalysed reaction between natural gas and steam. There is a need to import carbon dioxide or otherwise remove excess hydrogen to achieve the required ratio of 2 to 1 for the relative proportions of hydrogen and carbon monoxide in the resultant syngas. In many applications (including F-T processes, methanol synthesis and other chemical processes), such an opportunity to import carbon dioxide and/or export any separated excess hydrogen may not be available and/or economical;
(b) Partial oxidation (“POX”) of natural gas with pure oxygen which achieves a hydrogen to carbon monoxide ratio in the resultant syngas in the range from 1.6-1.8 to 1. Imported hydrogen is needed to achieve that required ratio of 2 to 1 for the relative proportions of hydrogen and carbon monoxide in the resultant syngas;
(c) Autothermal reforming (“ATR”) which uses a partial oxidation burner followed by a catalyst bed with a feed of natural gas, steam and oxygen to produce the required 2 to 1 ratio for the relative proportions of hydrogen and carbon monoxide in the resultant syngas; and
(d) Catalytic partial oxidation (“CPO”) which is the reaction of natural gas with oxygen over a catalyst that permits flameless partial combustion to hydrogen and carbon monoxide in the required relative proportions in the resultant syngas.
For POX, ATR and CPO, the oxidation reaction in the primary heat-generating unit is exothermic and, thus, the syngas is produced at elevated temperature. For example, POX produces syngas at a temperature of from 1200 to 1400° C., ATR produces syngas at a temperature of from 900 to 1100.degree. C. and CPO produces syngas at a temperature of from 1000 to 1100.degree. C.
The excess heat generated in these processes may be used to generate steam, for example in waste heat boilers, that can be used in steam turbines to generate power for air separation systems, air compressors and other equipment.
The excess heat may be used with additional natural gas and steam in a separate secondary unit to generate further syngas via steam-methane reforming. This process is the basis of the generic two-step HER process. In such a process, the high temperature syngas from the primary heat-generating unit is usually introduced to the shell-side of a shell and tube style steam-methane reformer. The tubes may contain conventional steam-methane reforming catalyst over which natural gas and steam react endothermically to form syngas. The heat from syngas on the shell-side of the reformer is used to drive the endothermic steam-methane reforming reaction. The syngas stream leaving the tubes can be separately collected and used to feed the primary exothermic syngas generator. Preferably, however, the syngas streams leaving the tubes are combined with the syngas on the shell-side to produce syngas having the desired ratio of hydrogen to carbon monoxide at a temperature of from 500 to 600.degree. C.
A secondary unit in which reforming takes place over catalyst using heat taken from the primary heat-generating unit is known as a Heat Exchange Reformer. One such example is described in U.S. Pat. No. 4,919,844 (Wang; published on 24 Apr. 1990) and is called an Enhanced Heat Transfer Reformer (or “EHTR”). The disclosure of this patent is incorporated herein by reference. Other existing HER processes are disclosed in WO-A-98/32817 (Halmo et al; published on 30 Jul. 1998), WO-A-00/09441 (Abbot; published on 24 Feb. 2000), WO-A-00/03126 (Fjellhaug et al; published on 20 Jan. 2000) and U.S. Pat. No. 5,362,453 (Marsch; published on 8 Nov. 1994). These disclosures are also incorporated herein by reference.
An example of an HER process is disclosed in U.S. Ser. No. 09/965,979 (filed on 27 Sep. 2001 and claiming priority from GB0025150.4 filed on 13 Oct. 2000) and this disclosure is incorporated herein by reference. In this example, a POX reactor is used in combination with an EHTR. Hydrocarbon fuel gas is reacted with steam and/or oxygen gas in a syngas generation system to produce a syngas product stream. An oxidant gas is compressed to produce a compressed oxidant gas, at least a portion of which is combusted in the presence of combustion fuel gas to produce combustion product gas. The combustion product gas is expanded to produce power and expanded combustion product gas. Heat from the expanded combustion product gas is recovered by using the expanded combustion product gas to heat steam by heat exchange to produce heated steam, at least a portion of which is used to provide at least a portion of any steam requirement for producing the syngas product stream in the syngas generation system. Additionally or alternatively, at least a portion of the oxygen gas is provided using an ASU that is driven by at least a portion of the power generated by the expansion of the combustion product gas.
Syngas product feeding conversion processes will unavoidably contain carbon dioxide. For F-T synfuel processes that use cobalt catalysts, this carbon dioxide behaves like an inert. Whilst it can be vented downstream, the carbon and oxygen capture efficiency of the entire gas to liquid (“GTL”) process is lower, which contributes to the greenhouse effect. It is thus desirable to recycle this carbon dioxide to the front-end syngas generator. It is a primary objective of preferred embodiments of this invention to enable efficient recycle of carbon dioxide and affect its efficient conversion to useful carbon monoxide, while minimizing the amount of such recycle and usage of oxygen feedstock.
Loss of carbon dioxide and methane from natural gas conversion processes is undesirable for several reasons. First, these gases are well known to have “greenhouse gas” properties. Secondly, valuable carbon atoms are being lost to the atmosphere thereby affecting the carbon efficiency and yield of the overall processes. Therefore, it is also an objective of preferred embodiments of the present invention to reduce the emission level of these greenhouse gases and other pollutants, for example oxides of nitrogen (“NO.sub.x”), and to recover at least some of the valuable carbon that is usually lost in natural gas conversion processes using HER technology for syngas generation.
In HER processes where hot gas is introduced to the shell-side of an HER, it is undesirable for the temperature of the syngas leaving the primary heat-generating unit to be too high as the mechanical integrity of the HER may be challenged. For example, the metal of the HER may lose its physical strength and soften. Therefore, it is another objective of preferred embodiments of the present invention to reduce or eliminate the possibility of problems with the mechanical integrity of the HER resulting from excessive syngas temperature in natural gas conversion processes using HER technology.
The POX process can generate syngas with small amounts of solid carbon particles or soot. This soot could foul or erode the heat exchange surfaces in the downstream HER. It is thus another objective of this invention to reduce or eliminate the potential for problems arising for such solid carbon particles.
U.S. Pat. No. 4,731,098 (Marsch: published on 15 Mar. 1988) discloses a reformer in which natural gas and steam are reformed to produce syngas. The syngas is then mixed with natural gas and oxygen or air before the mixture leaves the reformer.
Water has been used as a diluent in the production of syngas. Examples of such use of water have been disclosed by P. Osterrieth and M. Quintana (“A New Approach to the Production of Custom-made Synthesis Gas Using Texaco's Partial Oxidation Technology”; Texaco Development Corporation; AIChE meeting Presentation, 6 Mar. 1988) and by W. Francis Fong and M. E. Quintana (“HyTEX: A Novel Process for Hydrogen Production”; Texaco Development Corporation; NPRA 89th Annual Meeting, 17-19 Mar. 1991, San Antonio, Tex.)
U.S. Pat. No. 3,723,344 (Reynolds; published on 23 Mar. 1973) and U.S. Pat. No. 3,919,114 (Reynolds; published on 11 Nov. 1975) both describe processes for the generation of synthesis gas. The synthesis gas is produced by the partial oxidation of hydrocarbon fuel with a free oxygen-containing gas, optionally, in the presence of a temperature moderator such as steam. Carbon dioxide-rich gas or steam is combined with a stream of the synthesis gas product and the gaseous mixture is then subjected to a non-catalytic water gas reverse shift reaction and a portion of the carbon dioxide in the combined stream is reduced to carbon monoxide while simultaneously a stoichiometric amount of hydrogen is oxidized to water. Heat is removed from the resultant shift product gas in a waste heat boiler. Soot is then removed from the resultant cooled shift product gas using quench water in a gas-liquid contact apparatus. Carbon dioxide is then removed from the soot-depleted shift product gas and the resultant synthesis gas is then used in the synthesis of hydrocarbons and/or methanol.
In meeting the above-mentioned objectives, it is also important that any modifications to existing HER processes do not affect adversely the yield of conversion products, the capital and/or operating costs and the level of power usage.